1. Field of Invention
The invention relates to an image forming apparatus, such as a laser printer, that forms an image in an electrophotographic method and to a process unit removably set in the image forming apparatus.
2. Description of Related Art
A laser printer forms an image onto a sheet of paper in an electrophotographic method. More specifically, the laser printer includes a photosensitive drum, a charger, a scanner unit, a developing roller, and a transfer roller. As the photosensitive drum rotates, a surface of the photosensitive drum is uniformly charged by the charger. The scanner unit emits a laser beam based on image data. The laser beam scans across the charged surface of the photosensitive drum at high speed. Then, an electrostatic latent image is formed on the surface of the photosensitive drum. Toner carried on the developing roller is supplied to the electrostatic latent image, thereby developing the electrostatic latent image into a visible toner image on the surface of the photosensitive drum. When the sheet of paper passes between the photosensitive drum and the transfer roller, the toner image carried on the photosensitive drum is transferred onto the sheet by an electric field generated between the transfer roller and the photosensitive drum. In the above-described processes, the image is formed onto the sheet of paper.
In the laser printer, a rib that guides a sheet of paper between the photosensitive drum and the transfer roller is provided upstream of the transfer roller in a paper sheet feeding direction. The rib guides the paper sheet so as to bring the paper sheet into intimate contact with the surface of the photosensitive drum. Thus, the rib prevents a gap from being formed between the paper sheet and the photosensitive drum. If the gap is formed between the paper sheet and the photosensitive drum, electric discharge may occur at the gap due to a voltage applied to the transfer roller. However, such electric discharge can be prevented if the paper sheet is brought into intimate contact with the photosensitive drum, as described above.
However, when the visible toner image is transferred onto the paper sheet while being nipped between the photosensitive drum and the transfer roller, the paper sheet is pressed against the rib by a sheet transferring force at the nip portion. Consequently, the paper sheet continuously and frictionally slides on the rib while the paper sheet is conveyed. This makes the rib gradually wear down during a long period of usage of the laser printer. As the rib is worn, a gap is formed between the paper sheet and the photosensitive drum. When the electric discharge occurs at the gap due to the voltage applied to the transfer roller, poor image transfer is brought about.
An image forming apparatus of the invention may form a high-quality image over a long period of a usage thereof.
An image forming apparatus according to the invention may include a roller rotatably provided upstream of a transfer roller in a feeding direction of a recording medium. The roller guides the recording medium between a photosensitive drum and the transfer roller. As the roller is rotatably provided, friction between the recording medium and the roller may be reduced. Accordingly, wear or abrasion of the roller may be reduced. With the wear or abrasion of the roller reduced, a gap may not be formed between the photosensitive drum and the recording medium, so that electric discharge at the gap can be prevented.
It is preferable that a contact surface where the roller and the recording medium contact each other be disposed to a side of the photosensitive drum, with respect to a line tangent to a circumference of the photosensitive drum at an intersection of a line segment that links a center of the photosensitive drum and a center of the transfer roller.
By providing the contact surface where the roller and the recording medium contact each other, to the side of the photosensitive drum, the recording medium may be guided so as to come into intimate contact with the photosensitive drum. More specifically, the recording medium is brought into intimate contact with the surface of the photosensitive drum, so as to wind around a surface thereof before being fed between the photosensitive drum and the transfer roller. With such a structure, the occurrence of the electric discharge due to the bias applied to the transfer roller can be prevented. Accordingly, the poor image transfer can be prevented.
Even when the recording medium slides onto the contact surface of the roller while being pressed against the contact surface, friction generated between the roller and the recording medium may be reduced, because the roller is rotatably provided.
It is preferable that the photosensitive drum, the transfer roller, and the roller are provided in a process unit detachably set on the image forming apparatus, to facilitate maintenance of the image forming apparatus.
The photosensitive drum, the transfer roller, and the roller may be removed together, so that the positional accuracy of the photosensitive drum, the transfer roller, and the roller relative to each other may be improved. By the roller being mounted in improved positional accuracy, the recording medium may be properly guided so as to come into intimate contact with the photosensitive drum.
A contact surface where the recording medium and register rollers, provided upstream of the roller in a recording medium feeding direction, contact each other may be disposed in a lower portion, with respect to the contact surface where the roller and the recording medium contact each other.
With this structure, the recording medium receives a pressing force from the contact surface of the roller and is pulled between the transfer roller and the register rollers when the recording medium is held between the photosensitive drum and the transfer roller. The recording medium may be held between the photosensitive drum and the transfer roller without slack in the recording medium. Toner may be stably transferred onto the recording medium. Because the roller is rotatably provided, wear or abrasion of the roller due to sliding friction generated by the recording medium pressed against the roller can be reduced.
When the roller is structured so as to include a shaft and a plurality of disks provided in a lengthwise direction of the shaft with a predetermined distance therebetween, the roller having uniform dimensions may be formed by molding. Therefore, the recording medium may be properly guided by the roller accurately formed, so as to come into intimate contact with the photosensitive drum.
A supporting member that supports the recording medium guided by the roller may be provided between the transfer roller and the roller. The supporting member may be effective, especially when a trailing edge of the recording medium passes through the roller and comes out of the contact therewith, because the supporting member may support the trailing edge of the recording medium so as to prevent the trailing edge of the recording medium from slipping down between the transfer roller and the roller.
When a diameter of the roller is half a diameter of the transfer roller or smaller, the roller may be disposed at a position closer to a nip portion between the photosensitive drum and the transfer roller.
If the image forming apparatus of the invention is structured so as to collect the toner which remains on the photosensitive drum after an image transfer is finished, paper powders may possibly be collected together with the toner. However, by the use of the roller as in the invention, friction of the recording medium and the generation of the paper powders are reduced. That is, the mixture of paper powders into the toner to be collected can be prevented. Therefore, a reduction in the image quality can be prevented.
When the image forming apparatus is provided with a unit for forming an image on both sides of the recording medium, an electrical discharge is likely to occur due to the bias application when an image is to be formed on a side of the recording medium which already has an image formed on the other side thereof. However, by the use of the roller as in the invention, the recording medium may be guided so as to come into intimate contact with the photosensitive drum. Thus, the electrical discharge can be prevented.
The contact surface where the recording medium and register rollers contact each other may be disposed to a side of the transfer roller (lower side), with respect to the contact surface where the roller and the recording medium contact each other. In this case also, as described above, the recording medium receives a pressing force from the contact surface of the roller and is pulled between the transfer roller and the register rollers, through the roller. Toner may be stably transferred onto the recording medium.
With this structure, wear or abrasion of the roller due to sliding friction generated by the recording medium pressed against the contact surface of the roller may be reduced, because the roller is rotatably provided.
All of the photosensitive drum, the transfer roller, and the roller can be provided in a process unit. It is preferable that the process unit is detachably set in the image forming apparatus. By structuring the process unit so as to be detachably mounted on the image forming apparatus, the maintenance of the image forming apparatus is facilitated.
A high-quality image can be obtained by including the above-described features in the process unit.